[Dovecot] Bug in dovecot 2.2.5: segfault due to bad alignment
Timo Sirainen
tss at iki.fi
Thu Aug 29 01:44:08 EEST 2013
Attached patch fixes this? If yes, I'll commit it.
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On 21.8.2013, at 19.32, Andreas F. Borchert <dovecot at andreas-borchert.de> wrote:
> Take a look at the sources, hmac.h declares struct hmac_context:
>
> struct hmac_context {
> char ctx[HMAC_MAX_CONTEXT_SIZE];
> char ctxo[HMAC_MAX_CONTEXT_SIZE];
> const struct hash_method *hash;
> };
>
> If compiled for a 32 bit virtual address space, this has an alignment
> requirement of 4 due to the hash pointer.
>
> In line 171 of auth-token.c, we have following declaration of ctx
> as a local variable in auth_token_get():
>
> struct hmac_context ctx;
>
> This is put on an address with an alignment requirement of 4. In
> lines 174 and 175 hmac_init is invoked with hash_method_sha1:
>
> hmac_init(&ctx, (const unsigned char*)username, strlen(username),
> &hash_method_sha1);
>
> In hmac.c, lines 43 and following, ctx->ctx with an alignment of
> 4 is passed to meth->init and meth->loop where meth refers to
> hash_method_sha1:
>
> meth->init(ctx->ctx);
> meth->loop(ctx->ctx, k_ipad, 64);
>
> These functions refer now to sha1_init and sha1_loop where the
> first parameter is expected to be a pointer to struct sha1_ctxt,
> a data structure which is declared in sha1.h:
>
> struct sha1_ctxt {
> union {
> uint8_t b8[20];
> uint32_t b32[5];
> } h;
> union {
> uint8_t b8[8];
> uint64_t b64[1];
> } c;
> union {
> uint8_t b8[64];
> uint32_t b32[16];
> } m;
> uint8_t count;
> };
>
> Here we have with b64 one uint64_t which has on a SPARC platform
> an alignment requirement of 8. In consequence, struct sha1_ctxt
> has an alignment requirement of 8. With the invocations of
> meth->init and meth->loop above we pass a pointer to a data
> structure of alignment 4 to a function expecting a pointer to
> a data structure of alignment 8. Chances are that the alignment
> requirement is not met, causing a segmentation violation.
>
> This must be solved by declaring struct hmac_context such that
> is not just big enough but respects also the highest alignment
> required for one of the hashing data structures.
>
> There are several options to do this:
>
> * Beginning with C11, you are free to use an alignment specifier,
> i.e. add _Alignas ( uint64_t )
> (see section 6.7.5 in ISO 9899-2011)
>
> * GCC supports alignment attributes,
> i.e. add __attribute__ ((aligned (8)))
> or whatever is required instead of 8,
> see http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html
>
> * Do not use a local variable for it, allocate the data structure
> using malloc instead.
>
> If you want to see a live crash, here is the relevant output of gdb
> that debugs ${prefix}/libexec/dovecot/auth.
>
> Program received signal SIGSEGV, Segmentation fault.
> sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224
> 224 sha1.c: No such file or directory.
> (gdb) where
> #0 sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224
> #1 0xff2e218c in hmac_init (ctx=ctx at entry=0xffbff63c,
> key=key at entry=0x6a698 "borchert", key_len=8,
> meth=0x555d0 <hash_method_sha1>) at hmac.c:44
> #2 0x00023310 in auth_token_get (service=service at entry=0x6a648 "imap",
> session_pid=0x56071 "26272", username=0x6a698 "borchert",
> session_id=0x6a650 "AErv6nbk6gB/AAAB") at auth-token.c:174
> #3 0x00021708 in userdb_callback (result=USERDB_RESULT_OK, request=0x6a530)
> at auth-request-handler.c:668
> #4 0x0001f144 in auth_request_userdb_callback (result=<optimized out>,
> result at entry=USERDB_RESULT_OK, request=request at entry=0x6a530)
> at auth-request.c:1039
> #5 0x000312c8 in prefetch_lookup (auth_request=0x6a530,
> callback=0x1f058 <auth_request_userdb_callback>) at userdb-prefetch.c:40
> #6 0x0001f37c in auth_request_lookup_user (request=0x6a530,
> callback=callback at entry=0x2150c <userdb_callback>) at auth-request.c:1072
> #7 0x00022034 in auth_request_handler_master_request (
> handler=<optimized out>, master=master at entry=0x6b120, id=1292369921,
> client_id=1, params=0x55bfc) at auth-request-handler.c:758
> #8 0x0001be98 in master_input_request (args=<optimized out>, conn=0x6b120)
> at auth-master-connection.c:127
> #9 auth_master_input_line (line=<optimized out>, conn=0x6b120)
> at auth-master-connection.c:598
> #10 master_input (conn=0x6b120) at auth-master-connection.c:653
> #11 0xff2ecca4 in io_loop_call_io (io=io at entry=0x6b398) at ioloop.c:387
> #12 0xff2ed604 in io_loop_handler_run (ioloop=ioloop at entry=0x5e5e8)
> at ioloop-poll.c:211
> #13 0xff2ec7a8 in io_loop_run (ioloop=0x5e5e8) at ioloop.c:406
> #14 0xff29ad7c in master_service_run (service=0x5e128,
> callback=0x27b40 <client_connected>) at master-service.c:566
> #15 0x0001852c in main (argc=1, argv=0xffbffd54) at main.c:393
> (gdb) print ctxt
> $1 = (struct sha1_ctxt *) 0xffbff63c
>
> As you can see, ctxt is on a 4-byte boundary, not on an 8-byte boundary.
> The crash happens at sha1.c:224 where the 8-byte-alignment is indeed
> mandatory on a SPARC architecture:
>
> ctxt->c.b64[0] += copysiz * 8;
>
> The environment is Solaris 10 on SPARCv9. The sources have been compiled
> using gcc 4.8.0 for 32 bit.
>
> Andreas.
>
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